Method for controlling a connecting relay in a radio communications system

ABSTRACT

A radio communications system has a plurality of transmitter/receiver stations. At a certain point in time, a first set of said transmitter-receiver stations forms a group of active stations, which communicate with a mobile terminal device. When a requirement is detected, after a connection has been relayed, the number of active of stations is increased by at least one additional station. Thereafter, at least the station having the lowest transfer quality is removed from the active stations in order to close the connecting relay.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on and hereby claims priority to PCTApplication No. PCT/DE00/04535 filed on Dec. 19, 2000 and GermanApplication No. 199 615 16.0 filed on Dec. 20, 1999, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method for controlling a handover in a radiocommunications system.

A mobile radio communications system typically comprises a multiplicityof transceiver stations or base stations which exchange radio signalswith mobile terminals which are located within the range of thesestations, and at least one administration unit which switches user databetween mobile terminals which are located within the range of differentbase stations, or between a mobile terminal and a fixed network.

A “W-CDMA” mobile radio communications system such as the TDD mode ofthe forthcoming third-generation UMTS mobile radio system (UniversalMobile Telecommunication System) provides the facility for a mobileterminal to communicate at a given time with a subset of thesetransceiver stations by receiving user data associated with a singlecall connection on different channels of a plurality of these stations,and, conversely, user data transmitted by the terminal are also receivedby this plurality of stations. This subset is also designated as theactive set.

The purpose of this intrinsically redundant transmission is theavoidance of transmission gaps if the mobile terminal moves out of rangeof a station with which it is communicating; if it is communicating withone single station only, the radio link is interrupted when it movesoutside its range and the user data transmission from and to theterminal cannot continue until a different station is allocated to saidterminal, the user data which are to be transmitted to the terminal areswitched to this station, and the terminal and the new station can besynchronized with one another. On the other hand, in the event ofsimultaneous communication with a plurality of stations, failure of theradio link between the mobile terminal and one of the stations does notyet result in interruption of the transmission, since said transmissioncontinues to run unchanged via the other stations, while an alternativestation is defined for the failed station and communication is set upwith said alternative station. This type of station changeover withcontinuation of the communication with one or more other stations isreferred to as “soft” handover.

However, simultaneous communication with a plurality of stations entailsa substantial loading of the switching and transmission capacity of theradio communications system. If each active terminal occupiestransmission channels of a plurality of stations, the number of mobileradio subscribers which can be served simultaneously with a givennetwork infrastructure is of course considerably smaller than if eachterminal uses only one channel of a station. Similarly, in the mobileterminal, the need to process receive signals from a plurality ofchannels can result in increased power consumption and therefore in areduction of the network-independent usage time of a terminal of thistype. A compromise between transmission reliability and transmissioncapacity therefore needs to be found, which is usually such that, for aradio communications system of this type, the maximum number of stationswhich may belong to the active set is defined as a small value of e.g. 2or 3.

The stations which belong to the active set are identified usingregularly repeated evaluations of the quality of transmission betweenthe mobile terminal and the stations which are able to communicate withit. For this purpose, the mobile terminal measures the quality of radiosignals which it receives from these stations. Stations which do notbelong to the active set, but whose transmission quality, from oneevaluation to the next, has become higher than that of a station of theactive set, are reported to an administration unit which adds them tothe active set and in return excludes the poorer station.

Simulation experiments have shown that, in a system of this type, it mayoften be the case that the quality of transmission between the mobileterminal and all stations of the active set becomes so poor between twotransmission quality evaluations that the entire active set must bereplaced at once. This results in a transmission gap. Such a situationmay arise in particular if the mobile terminal is moving in an urbanenvironment, where roads bordered by buildings can transport a radiosignal over long distances in a longitudinal direction, but will screenit in a transverse direction, so that a terminal located on a road ofthis type may have an active set in which only relatively far-distantstations are located. As soon as the terminal moves into a transverseroad, transmission from and to all the stations may be virtuallysimultaneously interrupted.

From WO99/04593, a method is known for the selection of base stationsfor communication with a mobile station, in which the mobile stationreceives signals from a plurality of base stations, referred to as the“candidate set”, identifies a relevant receive strength and compares itwith a first threshold value. Base stations whose receive strengths lieabove the threshold value are reported to a base station controller assuitable for inclusion in the active set of base stations. The mobilestation identifies the need for changes to the current active setthrough measurements of the energies of pilot signals of the basestations of the active set and the candidate set, and through dynamicadaptation of the threshold values.

One potential object for the invention is to provide a method forcontrolling a soft handover with which high immunity of the transmissionto interruptions is achieved without loading the switching andtransmission capacity due to substantially increased redundancy, andwithout resulting in a significant increase in the power requirement ofthe terminal.

SUMMARY OF THE INVENTION

While the mobile terminal is located in a designated typical normaloperating condition, a check is preferably carried out periodically toascertain whether there is at least one station among the transceiverstations which do not belong to the active set which is suitable as anadditional station in the event that a handover must be carried out. Thecheck may be carried out at a frequency of at least 1 Hz. A station ofthis type, referred to below as a candidate station, is added to theactive set if a need for a handover is established. The periodicrepetition of the check is required in order to ensure that thecandidate station is still suitable if this need actually arises.Communication between the candidate station and the mobile terminal canthen be set up immediately without losing valuable time in identifying asuitable candidate station.

According to a first alternative, in order to identify the candidatestations, the transmission quality of a radio signal originating fromthe mobile terminal can be measured at the transceiver stations which donot belong to the active set, and the stations which receive the bestsignal are selected as the candidate stations. In such a case, it isappropriate for the measured transmission qualities to be transferredfrom the stations to an administration unit of the radio communicationssystem, which selects the candidate stations on the basis of thetransferred transmission qualities.

According to a second alternative, the mobile terminal can measure thetransmission quality of radio signals which originate from transceiverstations which do not belong to the active set, and one or moretransceiver stations which reveal the highest transmission quality areselected as candidate stations. The selection can be carried out by themobile terminal and reported to an administration unit of the radiocommunications system, or the mobile terminal transmits the measurementresults to the administration unit, which selects the candidatestations.

This second alternative can usually be implemented at lower cost thanthe first. Since an administration unit is normally responsible for alarge number of terminals, substantial computing outlay may be requiredin the administration unit in the first alternative in order to allocatethe transmission qualities delivered by the stations for each individualmobile terminal to the individual terminals, to compare them and toselect suitable candidates. If, however, according to the secondalternative, the transmission qualities are measured by the mobileterminal, the expensive allocation is not required, thereby relievingthe load on the administration unit.

The need for a handover is preferably identified if the qualities oftransmission between the mobile terminal and the transceiver stations ofthe active set fall below a limit value. The transmission qualityconsidered here may be that of the uplink transmission (from the mobileterminal to the station) and/or that of the downlink transmission (fromthe station to the mobile terminal).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome more apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

FIG. 1 shows a block diagram of a radio communications system, inparticular a mobile radio system,

FIG. 2 shows the results of an evaluation of the quality of transmissionbetween a mobile terminal and the individual stations of a radiocommunications system;

FIGS. 3 a to 3 c show a sequence of evaluation results at differenttimes during and after a handover procedure;

FIGS. 4 and 5 in each case show evaluation results at the beginning ofhandover procedures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings, wherein like reference numerals refer to like elementsthroughout.

The mobile radio system shown in FIG. 1 as an example of a radiocommunications system comprises a multiplicity of mobile switchingcenters MSC, which are networked with one another or which set up accessto a fixed network PSTN. Furthermore, these mobile switching centers MSCare in each case connected to at least one terminal RNC for allocationof radio resources, previously referred to as the administration unit.Each of these terminals RNC in turn enables a connection to at least onebase station BS. A base station BS of this type can set up a connectionvia a radio interface to subscriber stations, e.g. mobile stations MS orother mobile and stationary terminals. At least one radio cell is formedby each base station BS. An operation and maintenance center OMCimplements monitoring and maintenance functions for the mobile radiosystem or for parts thereof.

The functionality of this structure can be transferred to other radiocommunications systems. Examples of such systems are the GSM and FDDmode of the UMTS mobile radio system, or mobile radio systems based onthe American standard IS-95 with CDMA subscriber separation. One aspectof the invention relates in particular to W-CDMA systems, but isgenerally applicable to any radio communications system which offers thefacility for a plurality of radio links to an individual terminal to bemaintained, on which a plurality of stations in each case transmit thesame user data to the terminal or user data transmitted by the terminalare received and processed by a plurality of stations.

In order to explain different designs of the method, a radiocommunications system with a plurality of base stations BS1, BS2, . . .and an administration unit is considered. A mobile terminal MS moves inthe geographical area covered by the base stations, at differentdistances from the individual base stations and with differenttransmission qualities between it and the base stations.

FIG. 2 shows in diagrammatic form the transmission qualities between themobile terminal MS and five base stations BS1, BS2, BS3, BS4, BS5. Theminimum transmit power p which the mobile terminal MS requires in orderto transmit to the relevant base stations BS1 . . . BS5 whilemaintaining a given error quota is plotted on the vertical axis of thediagram as a measure of the transmission quality. The BS with the lowestminimum transmit power, i.e. the lowest on the diagram, is in each caseregarded as the BS with the highest transmission quality.

In the diagram in FIG. 2, BS1 has the highest transmission quality. Itsminimum transmit power forms the lower limit of a tolerance intervalreferred to as the handover margin HO, which, in a normal operatingcondition of the radio communications system, has a width of e.g. 5 dB.Further base stations BS2, BS3, BS4, BS5 lie within this interval. Anyavailable base stations with a higher minimum transmit power are notshown.

In the radio communications system considered here as an example, theactive set comprises a maximum of 2 elements in the normal operatingcondition, i.e. a call connection of the mobile terminal runssimultaneously via a maximum of two base stations, which transmit thesame user data to or receive the same user data from the mobileterminal. In FIG. 2, the active set comprises the stations BS1 and BS2shown by a solid black dot, which require the lowest transmit power.

At regular intervals, the mobile terminal checks the transmissionquality of the base stations in whose range it is located, e.g. bymeasuring the receive field strength of synchronization signals whichthe base stations continuously emit. This check may remain restricted tostations whose receive field strength on the mobile terminal does notfall below a given percentage of the receive field strength of thestrongest station BS1. These stations are generally identical to thosein the HO margin. In the case of FIG. 2, a check of this type indicatesthat, after the stations BS1 and BS2 of the active set, stations BS3 andBS4 are the next weakest stations. The mobile terminal reports these twostations to the administration unit as candidate stations which would besuitable replacements for the stations BS1 and BS2, if the connection tothe latter were to be interrupted. The candidate stations are in eachcase shown as semi-solid black dots, whereas the weakest station BS5 isshown by an unshaded circle in the diagram.

While the mobile terminal is moving, the strength ratios of theindividual stations may shift in relation to one another. If, forexample, the receive signal from BS3 becomes stronger than that fromBS2, BS2 is excluded from the active set and is replaced by BS3. Toavoid having to change the composition of the active set too frequentlyin the event of substantially changing receive conditions, a hysteresisis provided, whereby receive signals are replaced by one another only ifthe receive signal of the station which is getting stronger is, forexample, 1 dB stronger than that of the station which is getting weaker.

In this way, it is ensured in the case of most applications that theactive set constantly contains base stations with adequately hightransmission quality, so that the occasional exclusion of a station fromthe active set and its replacement by a differentstation does notadversely affect the communication of the mobile terminal.

If the quality of transmission between the mobile terminal and thestations of the active set is generally poor, situations may easilyarise in which, within a time interval between two transmission qualitychecks, the connection to all stations of the active set is interrupted.This risk can be reduced by allowing a larger active set, i.e. byproviding multiple-redundancy transmission from and to the mobileterminal. However it is clear that a solution of this type wouldseverely restrict the total capacity of a radio communications system,and would therefore increase costs for operators and users.

FIGS. 3 a to 3 c shows a situation of this type, in which the minimumtransmit powers of all stations BS1 to BS4 are clearly higher than inthe case shown in FIG. 2. The periodic checking of the transmissionquality, which can be carried out in the stations of the active set inparticular by measuring the bit error rate or the signal interferenceratio, delivers critically poor values, so that a slight furtherdeterioration could result in interrupted communication. In thissituation, the mobile terminal transmits a special warning signal, whichis intercepted by the stations of the active set BS1 and BS2 andforwarded to the administration unit. The radio communications systemthen switches to a temporary operating condition in which the number ofstations which may belong to the active set is increased, whereby theadministration unit adds the candidate stations BS3 and BS4 to theactive set and transmits a related message back to the mobile terminal.The mobile terminal then also begins, in addition to the channels usedby BS1 and BS2, to process receive signals on those channels which theadministration unit has allocated to the base stations BS3 and BS4, andto reconstruct the user data intended for it from the signals receivedon these four channels.

FIG. 3 b shows this temporary operating condition, wherein all four basestations BS1 to BS4 are shown here as solid black dots:.

FIG. 3 c shows a situation at a later time, where, on the basis of thesituation shown in FIG. 3 b, the transmission quality of the basestations BS1 and BS2 has simultaneously deteriorated, whereas thetransmission quality of BS3 has clearly improved so that it is now thestrongest station and defines the position of the HO margin. Since BS3has belonged to the active set since the condition shown in FIG. 3 b,the mobile terminal has been able to communicate constantly via it, andthe deterioration in the transmission quality of BS1 and BS2 has notresulted in interrupted communications. A transmission quality checkcarried out at the time shown in FIG. 3 c has indicated that BS1 and BS2have moved out from the HO margin. The mobile terminal reports this tothe administration unit, which then excludes BS1 and BS2 from the activeset, i.e. user data intended for the mobile terminal are no longerforwarded to these stations. The active set then comprises only thestations BS3 and BS4, whereby the normal operating condition is restoredand a soft handover is completed.

If BS1 and BS2, in contrast to the situation shown in FIG. 3 c, werestill located within the HO margin, the return to the normal operatingcondition would be completed in that, following a predefined duration ofthe temporary operating condition of e.g. 10 seconds, the mobileterminal reports the two poorest stations to the administration unitduring a transmission quality check, whereupon the administration unitexcludes these stations from the active set.

The method can also be applied to radio communications systems whichprovide different base station transmit powers depending on the receivesituation on the mobileterminal. To control the transmit power, themobile terminals of such a system transmit commands in the form of

“TPC” (Transmit Power Control) bits to the base stations to cause themto reduce or increase their transmit power. If a base station receives acommand to reduce the transmit power from a mobile terminal in thetemporary operating condition, this is a sure indication that thetransmission quality of this terminal is again high. The reception of acommand of this type therefore also gives cause to return to the normaloperating condition.

FIG. 4 shows a different situation with critically poor transmissionconditions. BS1 is again assumed to be the strongest base station. Thestation BS2 still remains within its HO margin HO, whereas the nextweakest stations BS3 and BS4 lie outside. In its periodic transmissionquality check, the mobile terminal therefore finds no suitable candidatestations in the HO margin HO, and consequently also reports none to theadministration unit. This is uncritical as long as the transmissionquality of the base stations of the active set is high, i.e. there isstill no risk of sudden interruption of the communication with them. If,however, as in the situation considered here, the transmission qualityof the active set is also poor, the mobile terminal increases the HOmargin to e.g. 10 dB, reports this to the administration unit and beginsto look for candidate stations in this increased HO margin HO′. It findsthe stations BS3 and BS4 in the increased HO margin HO′ and reports themto the administration unit. If the administration unit then receives thewarning signal from the mobile terminal, it switches to the temporaryoperating condition by adding BS3 and BS4 to the active set. The returnto the normal operating condition takes place as described above.

FIG. 5 describes a situation in which the HO margin of the strongeststation BS1 contains no further station. Since, in the normal operatingcondition, only those stations which lie within the HO margin HO areincluded in the active set, the active set in this case is smaller thanthe maximum value of 2 which is used here as an example. If thetransmission quality of the station BS1 falls under a limit value, themobile terminal extends the HO margin, as described with reference toFIG. 4. In the extended margin HO margin HO′, it finds the station BS2and reports it as a candidate station. On receiving the warning signal,the administration unit switches to the temporary operating conditionand adds BS2 to the active set. The active set then contains twostations, i.e. no more than is also permitted in the normal operatingcondition. If both stations BS1 and BS2 lie in the original HO margin HOat the end of the temporary operating condition, the station BS2 remainsin the active set; all that is then required in order to return to thenormal operating condition is for the mobile terminal to reverse theextension of the HO margin HO.

Various permutations of the method described here are possible. Thus,for example, the changeover from the narrow to the extended HO margin HOcan be made dependent on an approval of the administration unit.

The maximum permissible number of base stations in the active set may begreater than 2. For the maximum number of candidate stations which arereported to the administration unit, a fixed value can be predefinedwhich is greater than 2, but can also be 1. Instead of this, however, amaximum number of stations which are permitted to belong to the activeset in the temporary operating condition can also be predefined. If theactive set does not attain its maximum permissible size in the normaloperating condition, correspondingly more candidate stations can then bemeasured and added in the temporary operating condition to the activeset.

Instead of a receive quality check by measurements on the mobileterminal, or in addition to these measurements, correspondingmeasurements can also be carried out on the base stations of the activeset and the temporary operating condition can be initiated if thesemeasurements reveal a signal power which is too low, a bit error ratewhich is too high or a signal interference ratio which is too low. Sinceeach base station can only measure its own transmission quality, themeasurement results are transmitted to the administration unit, which,with the knowledge of the transmission qualities reported by thestations of the active set and, where appropriate, by the mobileterminal, makes a decision on the transition to the temporary operatingcondition.

Furthermore, base stations can independently apply to the administrationunit as candidate stations for a specific terminal if they do not belongto its active set, but are able to receive its radio signal well. Here,it can be provided that a base station applies particularly if itstransmission capacity is poorly utilized. If a station of this type isadded if necessary to the active set, this loads the capacity of theradio communications system less than in the case of a heavily utilizedstation.

The invention has been described in detail with particular reference topreferred embodiments thereof and examples, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

1. A method for controlling a handover in a radio communications systemhaving a multiplicity of transceiver stations, comprising: communicatingwith a mobile terminal through a first set of transceiver stations,wherein the first set of transceiver stations forms an active set ofstations, and adding at least one additional station to the active setof stations during a handover, thereby forming an extended set, theextended set communicating with the mobile terminal and being formedonce a need for the handover has been established, wherein a periodiccheck is carried out among transceiver stations that do not belong tothe active set of stations, to determine whether within a handovermargin below a highest transmission quality there is at least onecandidate station that is suitable as an additional transceiver stationfor communication with the mobile terminal if a handover is initiated,and wherein if there is no candidate station within the handover margin,the handover margin is extended to include at least one candidatestation, which is added to the active set of stations to form theextended set.
 2. The method for controlling a handover in a radiocommunications system as claimed in claim 1, further comprising creatinga new set of transceiver stations following a delay, to communicate withthe mobile terminal, the new set being formed by excluding at least thetransceiver station of the extended set which has the lowesttransmission quality.
 3. The method for controlling a handover in aradio communications system as claimed in claim 1, wherein the periodiccheck for a candidate station is carried out with a frequency of atleast 1 Hz.
 4. The method for controlling a handover in a radiocommunications system as claimed in claim 1, wherein the transmissionquality of a radio signal transmitted by the mobile terminal to each ofthe transceiver stations that do not belong to the active set isidentified, and the transceiver station with a highest transmissionquality is selected as a candidate station.
 5. The method forcontrolling a handover in a radio communications system as claimed inclaim 4, wherein the transmission quality is evaluated with reference toat least one of a strength of a receive signal, a required transmitpower of the mobile terminal, a bit error rate and a signal interferenceratio.
 6. The method for controlling a handover in a radiocommunications system as claimed in claim 4, wherein the transmissionquality of each transceiver station not belonging to the active set istransmitted to an administration unit in the radio communicationssystem, and the administration unit selects the candidate station. 7.The method for controlling a handover in a radio communications systemas claimed in claim 6, wherein, once the need for the handover has beenestablished, the administration unit causes at least one candidatestation to set up communications with the mobile terminal.
 8. The methodfor controlling a handover in a radio communications system as claimedin claim 1, wherein the mobile terminal identifies the transmissionquality of radio signals which are transmitted by transceiver stationswhich do not belong to the active set, and at least the transceiverstation whose transmission quality exceeds a specific limit value isselected as a candidate station.
 9. The method for controlling ahandover in a radio communications system as claimed in claim 8, whereinthe mobile terminal performs a selection to select the candidatestation, and a result of the selection is transmitted to anadministration unit in the radio communications system.
 10. The methodfor controlling a handover in a radio communications system as claimedin claim 9, wherein, once the need for the handover has beenestablished, the administration unit causes at least one candidatestation to set up communications with the mobile terminal.
 11. Themethod for controlling a handover in a radio communications system asclaimed in claim 10, wherein the identification of the candidate stationis carried out with a frequency of at least 1 Hz.
 12. The method forcontrolling a handover in a radio communications system as claimed inclaim 1, wherein the need for a handover is identified if transmissionqualities between the mobile terminal and the transceiver stations ofthe active set fall below a specific limit value.
 13. The method forcontrolling a handover in a radio communications system as claimed inclaim 12, wherein the mobile terminal evaluates the transmission qualitybetween the mobile terminal and each transceiver station of the activeset and assesses the need for the handover, the mobile terminalcommunicates the need for the handover to the administration unit of theradio communications system.
 14. The method for controlling a handoverin a radio communications system as claimed in claim 12, wherein thetransmission quality is evaluated at each transceiver station of theactive set each transmission quality is reported to the administrationunit of the radio communications system, and the administration unitidentifies the need for the handover.
 15. The method for controlling ahandover in a radio communications system as claimed in claim 14,wherein the mobile terminal evaluates the transmission quality betweenthe mobile terminal and each transceiver station of the active set andassesses the need for the handover, the mobile terminal communicates theneed for the handover to the administration unit of the radiocommunications system.
 16. The method for controlling a handover in aradio communications system as claimed in claim 15, wherein thetransmission quality is evaluated with reference to at least one of astrength of a receive signal, a required transmit power of the mobileterminal, a bit error rate and a signal interference ratio.
 17. Themethod for controlling a handover in a radio communications system asclaimed in claim 1, wherein the extended set is formed for a limitedtime which ends when a command to reduce the transmit power is exchangedbetween the mobile terminal and one of the transceiver stations.
 18. Themethod for controlling a handover in a radio communications system asclaimed in claim 1, wherein the extended set is formed for a limitedtime which is ended by an administration unit, based on a reportreceived from the mobile terminal.
 19. The method for controlling ahandover in a radio communications system as claimed in claim 1, whereinthe extended set is formed for a limited time which ends after apredefined duration.
 20. A mobile communications network comprising: aplurality of base stations to communicate with a mobile terminal, eachof the plurality of base stations having a transmission quality, theplurality of base stations comprising: an active set, comprising thoseof the plurality of base stations that have a transmission qualitygreater than or equal to a minimum transmission quality wherein theactive set actively communicates redundantly with the mobile terminal; acandidate set, comprising those of the plurality of base stations thathave a transmission quality greater than or equal to the minimumtransmission quality but are not in the active set; an inactive set,comprising those of the plurality of base stations that have atransmission quality less than the minimum transmission quality; and achanging mechanism to add at least one base station from the candidateset to the active set to thereby form an extended set, the extended setcommunicating with the mobile terminal and being formed once a need fora handover has been established, the extended set being formed for alimited time, wherein a periodic check is carried out among transceiverstations that do not belong to the active set of stations, to determinewhether within a handover margin below a highest transmission qualitythere is at least one candidate station that is suitable as anadditional transceiver station for communication with the mobileterminal if a handover is initiated, and wherein if there is nocandidate station within the handover margin, the handover margin isextended to include at least one candidate station, which is added tothe active set of stations to form the extended set.
 21. The mobilecommunications network of claim 20, wherein each of the base stations inthe inactive set evaluates the transmission quality between itself andthe mobile terminal, wherein each of the base stations in the inactiveset reports the transmission quality to an administration unit, andwherein the administration unit selects a base station from the inactiveset for inclusion in the candidate set when the base station from theinactive set demonstrates a transmission quality within the handovermargin.
 22. The mobile communications network of claim 21, wherein thetransmission quality is evaluated based on a field strength, as measuredat the mobile terminal, of synchronization signals continuously emittedby the base stations.
 23. The mobile communications network of claim 20,wherein the mobile terminal measures the transmission quality betweenthe mobile terminal and the inactive set.
 24. The mobile communicationsnetwork of claim 23, wherein the measuring comprises measuring thereceive field strength of synchronization signals continuously emittedby the base stations.
 25. The mobile communications network of claim 23,wherein the mobile terminal selects a base station from the inactive setfor inclusion in the candidate set when the base station from theinactive set demonstrates a transmission quality within the handovermargin, and wherein the mobile terminal reports the selection to anadministration unit.
 26. The mobile communications network of claim 23,wherein the mobile terminal reports the results of the measuredtransmission quality to an administration unit, and wherein theadministration unit selects a base station from the inactive set forinclusion in the candidate set when the base station from the inactiveset demonstrates a transmission quality within the handover margin. 27.The mobile communications network of claim 20, wherein the handovermargin is a range from a minimum transmit power required for a best basestation signal to 5 db more than the minimum transmit power.
 28. Themobile communications network of claim 20, wherein a base stations inthe active set is replaced by a base station from the candidate set whenthe transmission quality of the replaced base station in the active setis less than the transmission quality of the base station in thecandidate set by 1 dB or greater.
 29. The mobile communications networkof claim 20, wherein the base stations in the candidate set aretransferred into the active set when the highest transmission quality ofthe base stations in the active set falls below a predeterminedthreshold.
 30. The mobile communications network of claim 29, whereinthe active set is restored to its original size when the highesttransmission quality of the base stations in the active set exceeds thepredetermined threshold.
 31. The mobile communications network of claim20, wherein a base station in the active set is transferred into theinactive set when the transmission quality of the base station fallsoutside of the handover margin.
 32. The mobile communications network ofclaim 20, wherein the mobile terminal transmits a command to one of theplurality of base stations controlling the base station's transmitpower.
 33. The method for controlling a handover in a radiocommunications system as claimed in claim 20, wherein the extended setis formed for a limited time which is ended by an administration unit,based on a report received from the mobile terminal.
 34. The method forcontrolling a handover in a radio communications system as claimed inclaim 20, wherein the extended set is formed for a limited time whichends after a predefined duration.